Zimmerman is going to have to folks to believe that, he had his gun on him, but it was holstered, and while he was getting the thrashing of his life, he was able to un-holster his weapon and strike Treyvon dead center in the chest from underneath him with 1 shot.
Now the thing is, at such close proximity, Trayvon would have GSR on his hoodie. I think how much so will tell us alot about this case. Now a 9mm doesn't put out as much, say a .45, but its going to be difficult to explain how he got off such a great shot at Trayvon's chest, at the angle Trayvon was supposedly on top of him. Just picture your own self laying down, and envision someone on top of you, they would not be chest to chest with you, it would be more of a 45-90 degree angle, especially someone who is 6'3, with longer arms. Now imagine how difficult in a struggle for you to release a gunshot, right in the center of their chest. IMO
Now think how much easier it would be... if instead of the person on top of you... if you were on top of that person. That's why the angle of the bullet as well will tell alot of this case. An upward angle, through his aorta, will support Zimmerman's claim. A perfect shot, straight through his chest, right out his back at the same angle, will suggest Trayvon was shot while laying down. IMO